The pulsation of the outer layers of the BET Cep-type variable BW Vul.

Abstract

Eleven high-resolution ultraviolet spectrograms of BW Vul were obtained by means of the IUE over 1.6 periods to compare the pulsation of this star as observed in the strong far-UV resonance lines with observations of the UV lines of photospheric origin. The radial velocity curve of the C IV lines is different from the photospheric one, showing that both the photosphere and the C IV layers are accelerated upwards impulsively. After about one hour, the acceleration decreases to zero, and then the atmosphere falls with a nearly constant downward acceleration of 2100 cm/sq s. The varying asymmetry indicates mass loss due to the pulsation of the atmosphere, superimposed on the continuous mass loss of the star.